Process for preparing silver halide emulsions containing gelatin derivatives



United States Patent 3,138,461 PROCESS FOR PREPARING SILVER HA'LHDE EMULSIONS CONTAINING GELATIN DE- RIVATIVES William H. Ryan, Carlisle, Mass., assignor to Polaroid Corporation, Cambridge, Mass, a corporation of Delaware No Drawing. Filed June 10, 1960, Ser. No. 35,122 5 Claims. (Cl. 96-94) This invention relates to photography and, more particularly, to novel photosensitive elements and processes for the fabrication of same. It is specifically concerned with the preparation of photographic emulsions having silver halide as the sensitive element and wherein the silver halide is prepared in dispersed form in gelatin. The gelatin is in the form of, or is converted during preparation of the silver halide into, a gelatin derivative which derivative comprises the reaction product of certain heterocyclic compounds specified hereinafter and gelatin. The thus-prepared gelatin derivative-silver halide dispersion possesses the properties of being coagulated by pH adjustment to form granules or agglomerates of silver halide-gelatin derivative. This invention also includes emulsions comprising the aforementioned gelatin derivatives, especially those emulsions formulated by redispersing the aforementioned silver halide-gelatin derivative granules in a gelatin or gelatin derivative matrix or vehicle.

The most common method of preparing photographic silver halide dispersions is by reacting a water-soluble silver salt, such as silver nitrate, with at least one Watersoluble halide, such as potassium bromide, preferably together with potassium iodide, in an aqueous solution of a gelatin colloid peptizing agent. The dispersion of silver halide thus formed contains water-soluble salts as a byproduct of the double decomposition reaction, in addition to an unreacted excess of either of the initial salts. It has been considered desirable in the case of photographic emulsions that the water-soluble salts present in the emulsion be reduced in concentration. It has been generally the conventional practice, where gelatin has been used as a protective colloid, to remove the soluble materials therefrom by setting the gel dispersion by means of chilling, noodling the so-set dispersion, and washing the noodles with cold water.

It is one object of the present invention to provide novel photosensitive silver halide gelatin photographic emulsions.

Another object of the present invention is to provide novel methods for the fabrication of silver halide gelatin photographic emulsions.

A further object of the present invention is to prepare certain specified dispersions of silver halides useful for the preparation of photographic silver halide gelatin photosensitive emulsions.

A still further object of the present invention is to prepare certain specified dispersions of silver halides from which water-soluble by-products may be readily removed.

A still further object of the present invention is to provide a convenient practical method of preparing washed silver halide dispersions in which the specified dispersing colloid need be but a small percentage of the total composition and comprises the reaction product of gelatin and a heterocyclic compound of the type specified hereinafter.

A still further object of the present invention is to provide a method of washing specified silver halide dispersions in which substantially all of the liquid may be removed, thus making for substantially complete removal of the water-soluble impurities therein.

3,138,461 Patented June 23, 1964 Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the product possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

The term gelatin as utilized herein is intended to signify the art-accepted designation of the photographic colloid binder derived from collagen and comprising a proteinaceous substance. It is also intended to include any other product substantially identical therewith, as for' example, where such product is produced synthetically.

The fact that certain specified derivatives may be pre pared from the gelatin employed as a peptizer for preparing dispersions of silver halide and that dispersions of silver halide in those derivatives are acid coagulable and will form silver halide-gelatin derivative granules when the dispersion is adjusted to the proper pH is disclosed in the following United States Patents: 2,614,928 and 2,614,929, issued October 21, 1952; and 2,728,662, issued December 27, 1955. This method avoids the limitations which have previously characterized the preparation of washed silver halide dispersions and is valuable for expanding the possible conditions for the manufacture of photographic silver halide emulsions of difierent speeds, contrast and other photographic characteristics. In addition, this method offers a simple means of preparing emulsions of high silver concentration, which is of value in the manufacture of photographic film bearing large quantities of silver per unit area.

The present invention is specifically concerned with gelatin derivatives prepared by reacting gelatin with specified heterocyclic compounds, set out hereinafter, at a pH of 8 to 11 or more, imparted by the addition of a suitable base, such as sodium hydroxide, and preferably at a temperature of 20 to 60 C. When the preparation of the gelatin derivative is carried out at a pH in the lower part of the range given, any suitable base may be used, such as ammonium hydroxide or sodium carbonate, as well as sodium hydroxide and the corresponding potassium compounds. In the upper-part of the range, the use of a stronger base, such as sodium or potassium hydroxide, is preferred. The reaction between the gelatin and the heterocyclic agent employed takes place within a few minutes and a silver halide dispersion comprising the reaction product, as the peptizing colloid, takes on the character that it may be coagulated, forming silver halidegelatin derivative granules, upon acidification of the mass. The insoluble material which results consists of silver halide grains enveloped by gelatin derivative and may be washed free of salts and other water-soluble materials which may be present. As these granules readily settle from the liquid with which they are in contact, merely decanting the liquid from the granules removes substantially all of the water-soluble salts therefrom. The granules may be subsequently subjected to washing or a still more effective removal of the water-soluble salts may be obtained by redispersing the granules, such as in a solution having a pH at which they will disperse, and again coagulating, by lowering the pH, and removing the liquid from the granules thus formed a second time. The co.- agulation of the silver halide dispersion in the gelatin derivative is effected by acidifying the mass, preferably to a pH within the range 2 to 4, whereby the granules of silver halide and gelatin derivative will separate out.

In carrying out the present invention, the silver halide is prepared in an aqueous solution of gelatin as the peptizer,,the gelatin being present in the form of the desired (J derivative or converted into the gelatin derivative prior to acidification thereof, by imparting a pH of 8 to 11 to the solution and then reacting the gelatin with the particular heterocyclic compound employed. The silver halide may be prepared by any of the normal methods for the preparation of silver halide dispersions, such as, for example, by introducing a stream of an aqueous solution of silver nitrate and a stream of an aqueous solution of at least one alkali-metal halide, preferably potassium bromide and potassium iodide, into a constantly agitated solution of the peptizing agent. Instead, however, the gelatin solution may be combined with one of the reactants and the other reactants may be introduced therein with stirring. After the silver halide has been formed and the gelatin has been converted into the desired gelatin derivative, any ripening operations considered advisable may be effected. Acid is then added to the dispersion to lower the pH to a coagulating pH preferably within the range of 2 to 4. The silver halide-gelatin derivative dispersion forms granules and quickly settles from the liquid portion of the mass. The coagulum thus formed may then be separated from the liquid by any one of several techniques. In the simplest form, the supernatant mother liquor may be removed from the receptacle in which the mass is contained by means of a siphon or by decantation.

For some types of emulsions, the coagulum may be used directly without further washing. Where it is desired to further reduce the content of soluble salts which may be present, any one of several methods may be employed. For instance, the coagulum may be rinsed by treatment with cool water, preferably with the pH thereof adjusted to the range of the isoelectric point of the gelatin derivative. The specific number of rinses which is desirable to reduce the salt content may be determined by experiment, but usually in this method 1 to 4 rinses are sufficient. As an alternative method, the coagulum may be redispersed in water at an elevated temperature using water and a small amount of alkaline material, such as sodium hydroxide or ammonium hydroxide, so that the pH of the mixture is of the order of 6 or higher. The temperature of the solution is raised to approximately 40 C., and the whole is stirred for a few minutes to effect redispersion of the coagulum. The gelatin derivative-silver halide granules may again be precipitated by the addition of an appropriate acid to reduce the pH to the coagulation point of the derivative. The coagulum will again settle and the separation from the mother liquor is effected as previously described. This redispersion and coagulation may be repeated as many times as is necessary. As a third method of washing, the coagulum may be redispersed in an aqueous solution at a pH below 2.0 so as to effect redispersion of the silver halide-gelatin derivative granules. The low pH may be obtained by the addition of a quantity of an appropriate acid, such as sulfuric acid. Recoagulation may then be effected by the addition of a suitable base, such as sodium or ammonium hydroxide, to raise the pH of the solution to the coagulation point of the derivative. The coagulum will settle and the separation of the supernatant liquor is carried out as previously described. The coagulum after the washing operation consists of silver halide, the gelatin derivative, any other water-insoluble materials which may be present, and a small amount of residual soluble salts dissolved in any water trapped in the coagulum.

In carrying out the method in accordance with the present invention, the silver halide may be first prepared in a normal gelatin solution according to standard techniques for the preparation of silver halide, and any ripening which may be desired may be carried out. The pH of the mass is adjusted to a value of approximately 8 to 11 or higher with a suitable base, for instance, sodium hydroxide or potassium hydroxide, and the heterocyclic reagent for formation of the gelatin derivative is added in suitable physical form, usually dissolved in a solvent such as water, methanol, acetone, etc. After a suitable time has elapsed for reaction between the reagent and the gelatin, the pH of the mass may be lowered to the coagulation region. In the alternative process, the gelatin derivative may be formed, as set forth above, and the silver halide prepared, according to standard techniques, in the thus formed gelatin derivative.

The reagents employed for reaction with gelatin consist of heterocyclic compounds, preferably comprising a 5 or 6 membered heterocyclic ring system, possessing at least one configuration or group comprising a chlorine or bromine atom substituted on a cyclic carbon atom vicinal to a cyclic nitrogen atom.

In a preferred embodiment the designated heterocyclic compound possesses one or more acid groups such as carboxylic or sulfonic groups. In addition, it is preferred that the designated heterocyclic compounds possess one or more ballasting groups, which ballasting groups preferably possess one or more of the previously mentioned acid groups.

Preferred heterocyclic compounds within the generic concept may be represented by the formula:

x v x wherein X represents a chlorine or bromine atom; Y represents the atoms necessary to complete a heterocyclic ring system, preferably a 5 or 6 membered heterocyclic ring system; Z represents an acid group, preferably a carboxylic or sulfonic acid group or hydrogen; R represents a ballast group; and m is 0 or 1.

Specifically preferred heterocyclic reagents are compounds of the formula:

wherein W represents a N or CH group; at least one X comprises a chlorine or bromine atom; and at least one X is a Z, R, or a RZ group and wherein Z and R have the same significance as previously designated.

The reaction product formed by contact of the heterocyclic reagent with gelatin is believed to be the result of a condensation reaction between the designated halogen atom or atoms of the heterocyclic compound and the amino groups of the gelatin molecule. The resultant de-arninated, predominantly acid polymer not only precipitates or dissolves at the desired pH conditions, but also when in solution retains protective and peptizing properties for the silver halide, and when precipitated remains bound to the silver halide in an appropriate form of curd or grain such that settling is rapid and removal of the supernatant liquid can be efficiently carried out.

It will be recognized that the precipitation rate is determined in part by the number of acid groups contained by the heterocyclic compound as Well as the number of, size of, and configuration of the ballasting groups substituted on the heterocyclic nucleus. The acid groups act to enhance the acid properties of the condensation reaction product and thereby provide an increased propensity for insolubilization or precipitation thereof in a strongly acid medium. The ballast groups provide increased molecular weight to the condensation reactions product facilitating precipitation thereof and, when possessed of one or more of the previously mentioned acid groups, additionally act to enhance the acidic properties of the product and the precipitation thereof in an acid medium.

As examples of suitable ballasting groups which may be substituted on the heterocyclic nucleus, to increase the molecular Weight of the gelatin derivative and thereby provide an increased propensity for insolubilization, mention may be made of: acyclic groups such as long chain alkyl radicals linked directly or indirectly to the heterocyclic nucleus and cyclic groups such as aromatic radicals of the benzene and naphthalene series linked directly or indirectly to the heterocyclic nucleus, and/ or a combination thereof. It will be recognized that the ballast group selected is only limited by the fact that the resultant gelatinous polymer must be soluble in an alkaline medium and coagulable in an acidic medium While retaining its silver halide peptizing and protective properties. The range of choice of ballast group or groups from which the specific group or groups are chosen to provide the desired properties, is therefore extensive.

Specifically preferred heterocyclic groups for use in the practice of the present invention comprise halogenodiazine compounds, preferably halogeno-l,3-diazine compounds (pyrimidine compounds), and halogen-triazine compounds, preferably halogeno-1,3,5-triazine compounds.

As illustrative examples of halogeno-pyrimidine compounds useful in the practice of the present invention, mention may be made of: 2,6-dichloropyrimidine; 2-chloropyrimidine; and Reactone dyes such as Reactone Black RL and Reactone Red 2B170%. Additional halogenopyrimidine compounds are disclosed in British Patent No. 822,047, published October 21, 1959.

As examples of halogeno-triazine compounds useful in the practice of the present invention, mention may be made of: Procion dyes such as Procion Brilliant Red 5B, procion Printing Green 56, Procion Brilliant Yellow HSG, Procion Yellow R, Procion Brilliant Red H7B, Procion Scarlet H3G, Procion Brilliant Red 2B; Cibacron dyes such as Cibacron Black BG, Cibacron Brilliant Orange G, Cibacron Brilliant Yellow 36, Cibacron Brilliant Red 3B, Cibacron Brilliant Rubine R, Cibacron Yellow R, Cibacron Scarlet 2G, Cibacron Blue 36; Cibalan dyes such as Cibalan Brilliant Red BL; and symmetrical triazines such as 2,4-dichloro-6-(m-aminobenzoic)-l,3,5- s-triazine, 2,4-dichloro-6-(N-sulfanilic acid, sodium salt)- 1,3,5-s-triazine and 2-chloro-4,6-bis-(N-glycinic)-1,3,5-striazine.

As examples of additional heterocylic compounds within the generic formula and useful in the practice of the present invention, mention may be made of heterocyclic compounds of the thiazole series, such as, for example, 2-chlorobenzothiazole.

One method of preparing the preferred halogeno-1,3,5- triazine reagents containing ballasting groups comprises reacting, preferably in the presence of a suitable halogen acid acceptor, such as for example, quinoline, anhydrous pyridine, etc., a primary or secondary amino group containing ballasting compound with a triazinyl halide, for example, cyanuric chloride. The amine is employed in the ratio of approximately 1 mole, per mole of triazine com pound, for each ballast group to be introduced. Reaction with the triazine compound proceeds roughly stepwise, for example, with cyanuric chloride, the first chloro group reacts at temperatures of about l0 C. to about 0 C. and the second chloro group at about 40 to 50 C. Thus, it will be apparent to one skilled in the art that this provides an efiective means of controlling the degree and nature of substitution of the triazine ring, thereby enabling production of halogeno-1,3,4-triazines possessing bal lasting groups of varying types and properties whereby to effectively control the coagulation rate of the ultimately desired gelatin derivative.

It will be readily appreciated that the last-mentioned procedure also is applicable for the introduction of ballasting groups into the preferred halogeno-diazine reagents by means of an analogous condensation reaction.

For the acid which is to be employed for reducing the pH to the coagulation point, any acid may be employed which will give the desired pH to the mass and which will not deleteriously affect the photographic material. Dilute aqueous sulfuric acid has been found to be particularly useful for this purpose. Nevertheless, other acids, such as phosphoric acid, may be employed if sulfuric acid is not readily available.

It is apparent that one advantage of the precipitation technique for the preparation of photographic emulsions described herein is that the concentration of silver halide in the final emulsion may be adjusted to any desired value without reference to the quantities of water, silver halide and gelatin employed in the initial reaction for the preparation of the silver halide dispersion. Thus, concentrated emulsions may be readily prepared. Dried emulsions may be prepared for storage and shipment without any necessity of evaporation of water or reduction of the water content of the dispersed silver halide and without the necessity of using concentrated salt solutions or organic solvents. The coagulum prepared by the use of gelatin derivatives is especially suitable for the preparation of dried emulsions, since it may be obtained in a state nearly free of excess water, and therefore the drying operation is simplified and rendered less expensive. The coagulum may be obtained in a suitable form for the purpose, and the dried emulsion may be readily dispersed in water containing appropriate alkali at any time.

The invention will be illustrated in greater detail in conjunction with the following specific example which sets out representative preparations of the novel emulsions of this invention, which, however, is not limited to the details therein set forth and is intended to be illustrative only.

In the interest of clarity and brevity, the exemplary procedures and reactants have been consolidated.

The pH of a 12% aqueous gelatin solution was raised to 10 with 10% sodium hydroxide. 7.5 cc. of a 1 to 2% solution of each of the heterocyclic compounds specifically designated hereinbefore, in an appropriate solvent (water, acetone, methanol, etc.) was added to a 7.5 cc. aliquot of the aqueous gelatin solution. The mixture was heated at 40 C. for thirty minutes. (With some compounds higher temperatures provide more rapid results.) (During this period it may be necessary to add more alkali to maintain the pH at 10.)

Subsequent to heating, the pH of the mixture was lowered to 6.0 with 10% sulfuric acid. A solution comprising 60.0 cc. of water, 8.0 grams of potassium bromide and 0.29 gram of potassium iodide was added to the mixture.

The resultant mixture was heated to 70 C. and a solution comprising 10 grams of silver nitrate in cc. of Water was added with constant mixing.

The resultant mixture was stirred at 70 C. for twenty minutes and then cooled to 10 C. The pH of the mixture was lowered to 3.5 with 10% sulfuric acid and let stand until the gelatin and silver halide precipitated together, leaving a clear or slightly turbid mother liquor. (Precipitation generally occurs promptly but may require fifteen minutes or longer.) The supernatant liquid was decanted and the precipitate separated. The precipitate was washed with distilled water until the desired pAg or conductivity was attained.

A solution of 8 grams of gelatin in 30 cc. of water was added to the precipitate and the mixture heated at 50 C. for fifteen minutes with stirring. The pH was adjusted to 5.7 to 6.

The resultant emulsion may be sensitized, the desired additives incorporated therein, and the emulsion coated and processed according to conventional procedures known in the emulsion manufacturing art.

The emulsions of this invention may be coated onto various types of rigid or flexible supports, for example, glass, paper, metal, polymeric films of both the synthetic types and those derived from naturally occurring products, etc. Especially suitable materials include paper;

aluminum; polymethacrylic acid methyl and ethyl esters; vinyl chloride polymers; polyvinyl acetals; polyamides such as nylon; polyesters such as the polymeric films derived from ethylene glycol-terephthalic acid; and cellulose derivatives such as cellulose acetate, triacetate, nitrate, propionate, butyrate, acetate-propionate, or acetatebutyrate.

The light-sensitive material of the photographic emulsion comprises a compound of silver, for example, one or more of the silver halides, of which silver chloride, silver bromide and silver iodide are examples. The preferred silver halide gelatin emulsion comprises a silver iodobromide gelatin emulsion. It will be understood that preferred silver halide gelatin emulsions of varying halide concentration may be advantageously employed.

The emulsions of the present invention may be chemically sensitized by any of the accepted procedures. For example, the emulsions may be digested with naturally active gelatin, or sulfur compounds can be added such as those described in U.S. Patents Nos. 1,574,944, 1,623,- 499 and 2,410,689.

The emulsions may also be treated with salts of the noble metals such as ruthenium, rhodium, palladium, irridium, and platinum, all of which belong to Group VIII of the periodic table of elements and have an atomic weight greater than 100. The salts may be used for sensitizing in amounts below that which produces any substantial fog inhibition, as described in U.S. Patent No. 2,448,060 and as anti-foggants in higher amounts, as described in US. Patents Nos. 2,566,245 and 2,566,- 263.

The emulsions may also be chemically sensitized with gold salts as described in U.S. Patent No. 2,399,083 or stabilized with gold salts as described in U.S. Patents Nos. 2,597,856 and 2,597,915.

The emulsions may also be chemically sensitized with reducing agents such as stannous chloride as described in U.S. Patent No. 2,487,850; amines such as diethylene triamine as described in U.S. Patent No. 2,518,698; polyamines such as spermine as described in U.S. Patent No. 2,521,925; or bis-(B-aminoethyl)sulfide and its water soluble salts as described in U.S. Patent No. 2,521,926.

The emulsions may also be stabilized with the mercury compounds of U.S. Patents Nos. 2,728,663, 2,728,- 664 and 2,728,665.

In addition the emulsions may be optically sensitized by the addition of optical sensitizing dyes according to procedures well known to the art.

Where desired, suitable anti-foggants, restrainers, accelerators, preservatives, coating aids, and/or stabilizers may be included in the composition of the emulsions.

Hardening agents such as inorganic agents providing polyvalent metallic atoms, especially polyvalent aluminum or chromium ions, for example potash alum and chrome alum [K Cr (SO .24H O] and inorganic agents of the aldehyde type such as formaldehyde, glyoxal, mucochloric acid, etc., the ketone type such as diacetyl, and the quinone type, may be incorporated in the emulsions according to procedures well known in the art.

The term photosensitive and other terms of similar import are herein employed in the generic sense to describe materials possessing physical and chemical properties which enable them to form usable images when exposed to actinic radiation.

Since certain changes may be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method of preparing silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and at least one water-soluble alkali metal n (.5 halide salt in an aqueous solution of gelatin, thereby forming a silver halide dispersion, reacting the gelatin with a heterocyclic compound of the formula:

wherein at least one X is a halogen atom selected flunl the group consisting of chlorine and bromine; at least one X is selected from the group consisting of R, Z, and RZ wherein R is selected from the group consisting of acyclic and cyclic groups and Z is selected from the group consisting of carboxylic acid and sulfonic acid groups; and W is selected from the group consisting of N and CH groups; to thereby form a derivative of said gelatin, which derivative renders the dispersion coagulable at a pH within the range of about 2 to 4, coagulating said dispersion by adjusting said dispersion to a pH within said coagulating pH range, thereby forming silver halide-gelatin derivative particles, and separating the thusformed silver halide-gelatin derivative particles from the liquid portion of the mass.

2. A method of preparing a photographic silver halide emulsion which comprises the steps of mixing together silver nitrate and at least one water-soluble alkali metal halide salt in an aqueous solution of gelatin, thereby forming a silver halide dispersion, reacting the gelatin with a heterocyclic compound of the formula:

wherein at least one X is a halogen atom selected from the group consisting of chlorine and bromine; at least one X is selected from the group consisting of R, Z, and R-Z wherein R is selected from the group consisting of alkyl and aryl groups and Z is selected from the group consisting of carboxylic acid and sulfonic acid groups; and W is selected from the group consisting of N and CH groups; to thereby form a derivative of said gelatin, which derivative renders the dispersion coagulable at a pH within the range of about 2 to 4, coagulating said dispersion by adjusting said dispersion to a pH within said coagulating pH range, thereby forming silver halide-gelatin derivative granules, separating the thus-formed silver halidegelatin derivative granules from the liquid portion of the mass, and mixing the thus-formed granules with an aqueous solution of a polymer selected from the group consisting of gelatin and gelatin derivatives.

3. A method of preparing a photographic silver halide emulsion as defined in claim 2, wherein said gelatin derivative is formed prior to the addition of said silver nitrate and said water-soluble alkali metal halide salts.

4. A method of preparing a photographic silver halide emulsion as defined in claim 2, including the step of washing said separated silver halide-gelatin derivative granules prior to mixing thereof with said polymer.

5. A method of preparing a photographic silver halide emulsion as defined in claim 2, wherein said heterocyclic compound is selected from the group consisting of 2,4- dichloro-6-(m-aminobenzoic)-1,3,5 s triazine; 2,4-dichloro-6-(N-sulfanilic acid, sodium salt)-l,3,5-s-triazine; and 2-chloro-4,6-bis-(N-glycinic)-1,3,5-s-triazine.

References Cited in the file of this patent UNITED STATES PATENTS ,929 Yutzy et a1 Oct. 21, 1952 ,466 Wolfson Sept. 20, 1955 62 Yutzy et al Dec. 27, 1955 11 Allen et a1 May 9, 1961 A. hh m. A. MAJ 

1. IN A METHOD OF PREPARING SILVER HALIDE DISPERSIONS, THE STEPS WHICH COMPRISE MIXING TOGETHER A WATER-SOLUBLE SILVER SALT AND AT LEAST ONE WATER-SOLUBLEALKALI METAL HALIDE SALT IN AN AQUEOUS SOLUTION OF GELATIN, THEREBY FORMING A SILVER HALIDE DISPERSION, REACTING THE GELATIN WITH A HETEROCYCLIC COMPOUND OF THE FORMULA: 